electrical forces from the other five protons (dark arrows) are all
pushing it out of the nucleus, they are not sufficient to overcome
the strong nuclear forces.
In a very heavy nucleus, q/3, a proton that finds itself near the
edge has only a few neighbors close enough to attract it significantly
via the strong nuclear force, but every other proton in the nucleus
exerts a repulsive electrical force on it. If the nucleus is large enough,
the total electrical repulsion may be sufficient to overcome the at-
traction of the strong force, and the nucleus may spit out a proton.
Proton emission is fairly rare, however; a more common type of ra-
dioactive decay^1 in heavy nuclei is alpha decay, shown in q/4. The
imbalance of the forces is similar, but the chunk that is ejected is an
alpha particle (two protons and two neutrons) rather than a single
proton.
It is also possible for the nucleus to split into two pieces of
roughly equal size, q/5, a process known as fission. Note that in
addition to the two large fragments, there is a spray of individual
neutrons. In a nuclear fission bomb or a nuclear fission reactor,
some of these neutrons fly off and hit other nuclei, causing them to
undergo fission as well. The result is a chain reaction.
When a nucleus is able to undergo one of these processes, it is
said to be radioactive, and to undergo radioactive decay. Some of
the naturally occurring nuclei on earth are radioactive. The term
“radioactive” comes from Becquerel’s image of rays radiating out
from something, not from radio waves, which are a whole differ-
ent phenomenon. The term “decay” can also be a little misleading,
since it implies that the nucleus turns to dust or simply disappears
- actually it is splitting into two new nuclei with the same total
number of neutrons and protons, so the term “radioactive transfor-
mation” would have been more appropriate. Although the original
atom’s electrons are mere spectators in the process of weak radioac-
tive decay, we often speak loosely of “radioactive atoms” rather than
“radioactive nuclei.”
Randomness in physics
How does an atom decide when to decay? We might imagine
that it is like a termite-infested house that gets weaker and weaker,
until finally it reaches the day on which it is destined to fall apart.
Experiments, however, have not succeeded in detecting such “tick-
ing clock” hidden below the surface; the evidence is that all atoms
of a given isotope are absolutely identical. Why, then, would one
uranium atom decay today while another lives for another million
years? The answer appears to be that it is entirely random. We
can make general statements about the average time required for a(^1) Alpha decay is more common because an alpha particle happens to be a
very stable arrangement of protons and neutrons.
Section 8.2 The nucleus 511